The present invention relates to a traction control system for controlling the slip of a driving wheel or wheels of a vehicle, and more particularly to a traction control system for controlling the slip of a driving wheel or wheels during acceleration or the starting up of the vehicle.
Generally, when the traction force of the driving wheels during acceleration or the starting up of the vehicle becomes greater than a friction force between a wheel and the road surface [(a coefficient of friction between the wheel and the road surface) x (load on a driving wheel by the vehicular weight (vehicular load))], the driving wheels slip. A slip rate .lambda. indicating a degree of such slip is obtained by the following equation (I), where V.sub.w denotes a circumferential speed of the driving wheels, and V denotes a vehicle speed (a circumferential speed of driven wheels). EQU .lambda.=(V.sub.w -V)/V.sub.w (I)
As shown in FIG. 6, the friction force between the wheel and the road surface (Namely, a limit value of the traction force of the driving wheels) varies with the slip rate .lambda., and the friction force represented on the vertical axis reaches its maximum value at a predetermined value .lambda..sub.0. While the friction force between the wheel and the road surface is a friction force in a forward direction (longitudinal direction) of the vehicle, a friction force in a lateral direction (side force) is decreased with an increase in the slip rate .lambda. as indicated by a dotted line in FIG. 6.
On the basis of this point, there has been proposed a control method for detecting a slip rate .lambda. which maximizes the longitudinal friction force between the wheel and the road surface to maximize the driving efficiency of the vehicle and also suppresses the decrease in the lateral friction force between the wheel and the road surface in order to prevent side slip of the vehicle, and for approximating the slip rate .lambda. to the predetermined value .lambda..sub.0. Specifically, in the conventional method, a lower limit value .lambda..sub.1 and an upper limit value .lambda..sub.2 of the slip rate .lambda. define a predetermined range including the predetermined value .lambda..sub.0, and .lambda..sub.1 and .lambda..sub.2 are set according to the vehicle speed V. The torque of the driving wheels is controlled by a driving wheel torque control device according to a value of the slip rate .lambda. calculated from the driving wheel speed V.sub.w and the vehicle speed V. Consequently the circumferential speed V.sub.w of the driving wheels is controlled and the slip rate .lambda. of the driving wheels is feed-back controlled within the predetermined range .lambda..sub.1 -.lambda..sub.2.
In the conventional method mentioned above, the vehicle speed V used in the calculation of the slip rate .lambda., in accordance with the equation (I), is obtained by an average (.omega..sub.LR +.omega..sub.RR)/2 of speeds .omega..sub.LR and .omega..sub.RR of the left and right trailing (or driven) wheels (rear wheels of a front-wheel drive type vehicle, for example), (V=.omega..sub.LR +.omega..sub.RR /2). While the driving wheels are those supplied with power, the trialing (or driven) wheels are those not supplied with power. For example, in a front wheel drive vehicle, the front wheels are driving wheels and the rear wheels are trailing wheels. Conversely, in a rear wheel drive car, the rear wheels are the driving wheels, while the front wheels are considered to be the trailing wheels. This method of calculating the vehicle speed V is intended to prevent an error in detecting the vehicle speed V due to inner wheel off-tracking whether the vehicle is turning left or turning right. However, a difference in tracking distance is generated due to the turning difference between the driving wheels (the front wheels in a front-wheel drive vehicle, for example) and the trailing wheels during the turning of the vehicle. Accordingly, even when the driving wheels are not slipping, the vehicle speed V obtained in the above manner is different from both the left and right driving wheel speeds .omega..sub.LF and .omega..sub.RF. As a result, if the slip rate .lambda. is calculated in accordance with equation I, a non-slip condition of .lambda.=0 is not obtained. Therefore in a conventional system, slip control of the driving wheels is carried out under the condition where the driving wheels are apparently slipping based upon the above calculated vehicle speed V. Furthermore, even when the slipping of the driving wheels is actually very small, it may be incorrectly determined that the slip is excessive, and thus the slip control following this determination is erroneously carried out.